Bacteria in their natural environments are continuously exposed to changing environmental conditions such as oxygen limitation. To survive and complete bacteria have developed methods to monitor the state of the environment and adapt accordingly. Understanding the processes of microbial persistence, competition, and colonization are of great importance when contemplating the use of beneficial microbes such as rhizobia in agriculture, or alternatively the control of harmful pathogens during infection and disease. The goal of this purpose is to identify and characterize novel regulatory pathways important in the response of the soil bacterium R. meliloti to oxygen limitation in order to examine the role of this response in survival, competition, and colonization. Towards this goal, a collection of previously isolated Tn5luxAB insertions fused to promoters expressed under microaerophilic conditions will be tested for regulation by the known oxygen sensing system, FixL/FixJ. Novel regulatory loci required for the oxygen-limitation response will be identified by second-site mutagenesis of strains containing Tn5luxAB fusions whose expression is FixL/FixJ independent. The importance of the novel regulatory loci and of fixL for survival and completion under microaerophilic conditions in nutrient-rich soil will be determined. A role for the loci in legume nodule formation and legume colonization will also be examined.